1. Field of the Invention
The present invention relates to a position sensor and to parts therefor. The invention has particular although not exclusive relevance to x-y digitising tablets which operate with a cordless stylus. The invention is particularly useful for embedding behind the display of a hand-held electronic device such as a personal digital assistant (PDA), mobile telephone, web browser or products embodying combinations of these.
2. Related Art
U.S. Pat. No. 4,878,533 discloses an x-y digitising tablet which uses a resonant stylus. The digitising tablet comprises a large number of overlapping, but separate, loop coils which are arrayed in the x-y direction. These loop coils are connected through a switching circuit and a multiplexing circuit to an excitation circuit and a receiving circuit. This system is arranged so that the multiplexing circuit connects each of the loop coils in sequence to the switching circuit which firstly connects the connected loop coil to the excitation circuit and then to the receiving circuit. When a loop coil is connected to the excitation circuit, a current is applied to the loop coil which energises the resonant stylus. When the loop coil is connected to the receiving circuit, the receiving circuit detects the electromotive force (EMF) induced in the connected loop coil by the resonant stylus. This system identifies the current position of the stylus by detecting the loop coil which provides the greatest output signal level.
A problem with the digitising tablet described in U.S. Pat. No. 4,878,533 is that it consumes a large amount of power in order to energise and detect the signals in each of the loop coils, which makes it unsuitable for hand-held battery-powered devices such as PDAs and mobile telephones.